Firstly, i kinda dislike the idea of walling features off behind playtime for whatever reason. It goes against the idea of this server in my eyes.
And secondly I honestly believe this may actually be counterintuitive. Harmful exploits can be done with such a large variety of items, many with legitimate purposes, that we'd have to block a seriously large amount of stuff to the point that legitimate players would be put off. And all that.. for 20 minutes? The people doing this could quite easily just afk for 20 minutes after joining or set up some bots to have a few alts log on for 20 min during quiet times. It'll make administration more difficult since it'll be harder to keep track of things - instead of keeping an eye on a suspicious player, we would have to set a timer for half an hour and then keep an eye on them.
And I do say so myself, but the situation that caused this suggestion was dealt with very well by the admins online at the time. We knew who to watch and we knew how to deal with the person - you can't ban someone for creating a crash chunk until they've made it. Not much more could've been done, and this suggestion is akin to disallowing block interactions for half an hour because we got a person nukering spawn a few times.
So yeah, I believe this will hurt player retention, make administration more complicated, go against the nature of the server and wouldn't even deal with the issue, so I object.